The present invention is related to conjugate fibers of two different thermoplastic polymers and nonwoven webs produced therefrom. More specifically, the invention is related to conjugate fibers and nonwoven webs of a polyolefin and a polyamide.
Conjugate fibers contain at least two component compositions that occupy distinct cross sections along substantially the entire length of the fibers, and they are produced by simultaneously and contiguously extruding a plurality of molten fiber-forming polymeric compositions to spinning orifices of a spinneret to form unitary filament strands. In general, component compositions for conjugate fibers are selected from different polymers having different shrinkage properties and/or complementarily advantageous chemical and physical properties. Component polymers having different shrinkage properties are typically utilized to impart crimpability in the conjugate fibers, and component polymers having different advantageous properties are utilized to impart different functionalities in the fibers.
Since different polymers have different melting and processing temperatures as well as have different rheological melt properties, it is usually necessary or desirable to process and maintain component polymer compositions for conjugate fibers at different temperatures until just prior to combining the melted polymer compositions as unitary filament strands. In many cases, when different melted polymers are combined to form unitary strands, there emerge numerous processing difficulties such as nonuniformity of the strands, spin breaks and bending phenomena of unsolidified strands at the tip of the spinneret. Such processing difficulties prevent proper formation of fibers and, thus, nonwoven fabrics. In addition, especially for fiber-spinning processes that employ pneumatic drawing steps, e.g., meltblown fibers and spunbond fibers, the filament strands exiting the spinneret tend to bundle or rope during the drawing process unless the processing conditions are carefully tailored for each polymer combination. Such controlled processing conditions ensure, for example, proper quenching of the component polymers forming the filament strands and separation of the extruded strands until they are deposited onto the forming surface. There have been many approaches to solve these processing difficulties in spinning conjugate fibers of different polymer compositions. For example, British Pat. No. 965,729 discloses a spinneret containing angularly placed orifices that is inclined in the opposite direction to the bending direction of the extruded conjugate fiber strands. However, the teaching of the patent may only be practical for large production runs since a specific spinneret has to be constructed for each combination of different polymers. U.S. Pat. No. 3,536,802 to Uraya et al. discloses a method of separately extruding and maintaining component polymer compositions at different temperatures until just prior to combining and extruding into unitary fiber strands in order to alleviate the processing difficulties by keeping the melt viscosities of the component polymer compositions substantially at the same level. The teaching of Uraya et al. utilizes the fact that linear thermoplastic polymers in general decrease their melt viscosity as the melt temperature increases. However, the process of ensuring the thermal profile of each component polymer melts requires a cumbersome or complex spinneret assembly that contains insulation layers. In addition, the temperature difference of the polymer components of the unitary filament strands creates processing difficulties in handling the extruded filaments. For example, component polymers having different melt temperatures tend to solidify at different rates, and insufficiently quenched polymer components of the conjugate filaments tend to cause random fusing or roping of the filament strands before the filaments can be properly deposited on a forming surface.
Of various conjugate fibers having different polymers, conjugate fibers of a polyolefin and polyamide combination are highly useful. U.S. Pat. No. 3,788,940 to Ogata et al., for example, discloses conjugate fibers containing a polyolefin and a long-carbon chain polyamide, e.g., nylon-11, nylon-12, nylon-11/10, nylon-11/11 or nylon-11/12. Long-carbon chain polyamides have melting and processing temperatures that are lower than more commonly available and conventional nylons, i.e., nylon 6 and nylon 6/6. The melting and processing temperatures of these long-carbon chain polyamides are practically comparable to those of polyolefins such that these polyamides and polyolefins can easily be processed to form conjugate fibers. In contrast, more conventional and economical polyamides, i.e., nylon 6 and nylon 6/6, have significantly higher melting points and, thus, have to be melt-processed at a higher processing temperature range than typical polyolefins. Moreover, as is known in the art, a thermoplastic polymer is typically melt-processed at a temperature significantly higher than the melting point of the polymer in order to accommodate the typical temperature fluctuation of the melt-processing equipment, e.g., an extruder, and, thus, to avoid accidental solidification or freeze up of the polymers in the melt-processing equipment and to provide a composition melt that has a properly processible melt viscosity. In general, when a composition melt is underheated, the melt has an elongational viscosity or a melt elasticity that is too high to allow proper drawing of the extruded filaments; and when a melt is overheated, the extruded filaments from the melt cannot be quenched properly and sufficiently. Consequently, overheated and underheated melts do not properly form useful filaments, e.g., cause spin breaks and form fused and/or roped fibers. Accordingly, conventional polyamides and polyolefins have been melt-processed at different processing temperatures and, therefore, typically required specialized processing equipment to produce conjugate fibers.
There remains a need for providing polymer compositions for polyolefin/polyamide conjugate fibers that can be processed with conventional polyolefin processing equipment and that contain polymer components which need not be processed at different processing temperatures.